1. Field of the Invention
The present invention relates to a voltage regulator capable of stabilizing an output voltage even when a power supply fluctuates.
2. Description of the Related Art
A related-art voltage regulator is now described. FIG. 9 is a circuit diagram illustrating the related-art voltage regulator.
The related-art voltage regulator includes an error amplifier circuit 103, a reference voltage circuit 102, PMOS transistors 901 and 902, an output transistor 105, resistors 106, 107, and 903, a fluctuation detection capacitor 904, a clamp circuit 905, a ground terminal 100, an output terminal 104, and a power supply terminal 101.
The resistors 106 and 107 are connected in series between the output terminal 104 and the ground terminal 100, and divide an output voltage Vout generated at the output terminal 104. A voltage generated at a connection point of the resistors 106 and 107 is represented by Vfb. The error amplifier circuit 103 controls a gate voltage of the output transistor 105 so that the voltage Vfb may approach a voltage Vref of the reference voltage circuit 102, to thereby control the output transistor 105 to output an output voltage Vout from the output terminal 104. When a power supply voltage VDD of the power supply terminal 101 increases, a current Ix1 is allowed to flow from the power supply terminal 101 to the fluctuation detection capacitor 904. The current Ix1 is amplified by a current feedback circuit including the PMOS transistors 901 and 902 and the resistor 903, to thereby generate a current Ix2. The current Ix2 is supplied to a gate of the output transistor 105 to charge a gate capacitance of the output transistor 105. In this manner, a gate-source voltage VGS of the output transistor 105 is adjusted to an appropriate value even when the power supply voltage VDD corresponding to a source voltage of the output transistor 105 fluctuates, and hence overshoot is suppressed to stabilize the output voltage Vout (see, for example, Japanese Patent Application Laid-open No. 2007-157071).
However, the related-art voltage regulator has a problem in that, when the power supply voltage continues to fluctuate even after the fluctuation in power supply voltage is detected to suppresses the overshoot of the output voltage, the voltage regulator continues to control the output transistor excessively to generate undershoot or another overshoot. Further, the related-art voltage regulator has another problem in that, when the power supply voltage fluctuates quickly under a heavy load and undershoot is generated after the overshoot of the output voltage is suppressed, the voltage regulator erroneously detects an operation of subsequently increasing the output voltage to control the output transistor, resulting in oscillation.